Question: Stephanie is 4 years older than Luis. Seven years ago, Stephanie was 3 times as old as Luis. How old is Luis now?
Explanation: We can use the given information to write down two equations that describe the ages of Stephanie and Luis. Let Stephanie's current age be $s$ and Luis's current age be $l$ The information in the first sentence can be expressed in the following equation: $s = l + 4$ Seven years ago, Stephanie was $s - 7$ years old, and Luis was $l - 7$ years old. The information in the second sentence can be expressed in the following equation: $s - 7 = 3(l - 7)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $l$ , it might be easiest to use our first equation for $s$ and substitute it into our second equation. Our first equation is: $s = l + 4$ . Substituting this into our second equation, we get the equation: $(l + 4)$ $-$ $7 = 3(l - 7)$ which combines the information about $l$ from both of our original equations. Simplifying both sides of this equation, we get: $l - 3 = 3 l - 21$ Solving for $l$ , we get: $2 l = 18$ $l = 9$.